JP3607569B2 - Nondestructive inspection method and apparatus for structure - Google Patents

Description

【００２３】
ここでκは熱伝導率、Ｄ＝κ／（ρＣ）は熱拡散率、ρは密度、Ｃは熱容量である。ただし、レーザーの発光中（０＜ｔ＜ｔ_０ ）においては（１）式の第１項のみを採る。
一方、光の透過距離が数１０μｍ以上ある絶縁体では、レーザー光のパルス幅程度の時間での熱拡散距離は相対的に小さいので無視できる。この場合、温度はレーザー光の照射時間に比例して上昇し、物体内部の温度分布Ｔ（ｘ_３，ｔ）は次式で与えられる。
【００２４】
【数２】

【００２５】
続いて、パルスレーザー光を照射したときの物体表層の温度分布を、従来からあるレーザー超音波法の場合と本発明の場合とで比較する。図３は、（１）式〜（３）式より求められた下記の条件における物体表層の温度分布を示すグラフである。このグラフの横軸は物体表面からの距離ｘ_３ ［μｍ］であり、縦軸はレーザー照射による温度上昇Ｔ［Ｋ］である。
【００２６】
まず、従来法による場合を検討するにあたり、パルス幅ｔ_０ ＝１０ｎｓの短パルスのレーザー光１１１ａを照射した場合の温度分布を金属と絶縁体の両方で調べる。
物体が金属の場合、熱物性定数は軟鋼を想定して熱伝導率κ＝５０Ｗ／（ｍ・Ｋ）、熱拡散率Ｄ＝１．３×１０^−５ｍ^２ ／ｓとする。ただし、光吸収係数γを無限大とする。また、吸収パワー密度Ｉ_０ はＱスイッチ方式により発振されたレーザー光を想定して２０ＭＷ／ｃｍ^２ （ビーム直径８ｍｍ、吸収エネルギー１００ｍＪに相当）とする。図３の実線ａは、パルス幅ｔ_０ の１０倍の時間ｔ＝１００ｎｓが経過したときの温度分布である。温度上昇Ｔは表面で２６４Ｋであるが、４μｍ内部では１０Ｋと小さい。常温（２９３Ｋ）の場合、照射直後の表面温度は融点（１８１０Ｋ）を超えるので、アブレーションが起こる。
【００２７】
物体が絶縁体の場合、熱伝導率κおよび熱拡散率Ｄは金属の場合と同じ値とし、光吸収係数γ＝１×１０^４ ［１／ｍ］とする。また、吸収パワー密度Ｉ_０ を金属の場合の１０倍の２００ＭＷ／ｃｍ^２ とする。図３の点線ｂは、レーザー発光が終了した時間ｔ＝１０ｎｓにおける温度分布である。最表面の温度上昇Ｔは５２Ｋでアブレーションは起こらず、１００μｍ内部でも２０Ｋ以上温度が上昇する。
【００２８】
次に、本発明による場合を検討するにあたり、パルス幅ｔ_０ ＝５０μｓの長パルスのレーザー光１１ａを照射した場合の温度分布を金属と絶縁体の両方で調べる。吸収パワー密度Ｉ_０ は１ＭＷ／ｃｍ^２ とする。
物体が金属の場合、パルス幅ｔ_０ の１０倍の時間ｔ＝５００μｓ経過後には、図３の破線ｃで示すように、厚さ１００μｍの表層で温度上昇Ｔが６０Ｋとなった。また、光が透過する絶縁体の場合、光透過と内部での吸収とにより、レーザー発光が終了した時間ｔ＝５０μｓには、図３の一点鎖線ｄで示すように、１００μｍ内部での温度上昇Ｔが５０Ｋとなった。いずれの場合も、表面での温度上昇Ｔは６００Ｋ以下であり、アブレーションは起こらない。
【００２９】
以上からわかるように、長パルスのレーザー光１１ａを用いた場合、金属でも絶縁体でも、比較的厚い表層に適度な温度上昇が起こる。したがって、比較的厚い表層に熱膨張による振動を励起できるので、大型構造物の内部にできた亀裂などの欠陥を検査できることがわかる。
その一方で、レーザー光１１ａはパルス幅ｔ_０ が長いので大エネルギーのわりにピークパワーＩ_０ が小さく、物体表面でアブレーションが起きない。このため、検査対象の表面にアブレーション損傷が無い完全な非破壊検査を行える。
【００３０】
また、レーザー光１１ａの吸収パワー密度Ｉ_０ は１ＭＷ／ｃｍ^２ 程度であり、図１に示した音響光学的変調器１２の耐光性限度以下である。したがって、音響光学的変調器１２の表面をアブレーション損傷させることなく使用できる。音響光学的変調器１２を用いることにより、レーザー光１１ａの波形を自在に制御できる。特に、音響光学的変調器１２はレーザー光１１ａの偏向を機械的でなく光学的に行うので、高速に走査可能となる。なお、言うまでもなく、走査手段として反射ミラーを用いることもできる。
【００３１】
次に、長パルスのレーザー光１１ａを出力するレーザー光源１１について更に説明する。
図４は、レーザー光源１１の構成図である。レーザー光源１１は、固体レーザーであり、レーザーヘッド部２０と、電源部３０と、レーザー発振制御部５１と、冷却部５２とから構成されている。
さらに、レーザーヘッド部２０はノーマル発振の固体レーザー発振器であり、レーザーロッド２１と、フラッシュランプ２２と、レーザーロッド２１およびフラッシュランプ２２を収容する集光器２３と、光共振器を構成する全反射鏡２４ａおよび出力鏡２４ｂと、集光器２３の一方の鏡面と全反射鏡２４ａとの間に配置されたエキスパンダー（広がり角制御手段）２５と、集光器２３の他方の鏡面と出力鏡２４ｂとの間に配置されたポラライザー２６とを備えている。
【００３２】
レーザーロッド２１には、Ｎｄ：ＹＡＧ結晶、Ｅｒ：ＹＡＧ結晶、Ｎｄ：ガラス結晶、Ｎｄ：ＹＬＦ結晶などの固体レーザー媒質が用いられる。フラッシュランプ２２は、レーザーロッド２１に光を照射して励起させる励起ランプであり、キセノンフラッシュランプなどが用いられる。集光器２３は、フラッシュランプ２２からの光を効率よくレーザーロッド２１上に集光するものである。
【００３３】
エキスパンダー２５は、凸レンズ２５ａと凹レンズ２５ｂとから構成されている。そして、各レンズ２５ａ，２５ｂの屈折率と、各レンズ２５ａ，２５ｂ間の距離を調整することで、レーザーヘッド部２０（すなわち、レーザー光源１１）から出力されるレーザー光１１ａの広がり角を制御できる。ここでは、指向性を高くするため、レーザー光１１ａの広がり角を１０ｍｒａｄ以下に制限する。
ポラライザー２６は、レーザー光１１ａを直線偏光とするための偏光素子である。このポラライザー２６を用いて直線偏光出力を得ることにより、図１に示した音響光学的変調器１２により高効率な変調と偏向が可能となる。なお、この音響光学的変調器１２を用いない場合には、ポラライザー２６は無くてもよい。
【００３４】
電源部３０は、レーザーヘッド部２０のフラッシュランプ２２に放電電流を供給する電源である。レーザー発振制御部５１は、電源部３０に接続されており、この電源部３０がフラッシュランプ２２に放電電流を供給するタイミングを制御するものである。冷却部５２は、レーザーヘッド部２０のレーザーロッド２１、フラッシュランプ２２および集光器２３を冷却（例えば水冷）するものである。
【００３５】
図５は、図４に示した電源部３０の構成を示す回路図である。
フラッシュランプ２２の陽極はダイオード３３および抵抗３２を介してシマー電源３１の＋端子に接続され、陰極はこのシマー電源３１の−端子に接続されている。また、フラッシュランプ２２の陽極はスイッチング素子４０ａを介して放電用コンデンサー３９ａの一端に接続され、陰極はこの放電用コンデンサー３９ａの他端に接続されている。さらに、フラッシュランプ２２の陽極はインダクター４１およびスイッチング素子４０ｂを介して放電用コンデンサー３９ｂの一端に接続され、陰極はこの放電用コンデンサー３９ｂの他端に接続されている。
【００３６】
放電用コンデンサー３９ａ，３９ｂはケース３９ｃに収容されている。スイッチング素子４０ａ，４０ｂのそれぞれのゲートはレーザー発振制御部５１に接続されている。
そして、放電用コンデンサー３９ａ，３９ｂのそれぞれの一端はインダクター３８ａ，３８ｂを介して充電用主電源３７の＋端子に接続され、それぞれの他端はこの充電用主電源３７の−端子に接続されている。
また、電源部３０は、トリガートランス３５を介して電極３６よりフラッシュランプ２２にトリガーパルスを印加するトリガーパルス発生器３４を有している。
【００３７】
シマー電源３１は、フラッシュランプ２２に常時所定の直流電流を供給する予備放電回路を構成する。このシマー電源３１の両端子（＋，−）間の電圧は、充電用主電源３７の両端子（＋，−）間の電圧よりも低く設定してある。
放電用コンデンサー３９ａとスイッチング素子４０ａとにより、フラッシュランプ２２にパルス電流を供給する高速放電回路が構成される。ここで、スイッチング素子４０ａは、通常はアノード電流の通過を阻止するが、ゲートに発光指令信号５１ａが印加されるとアノード電流を通過させる素子である。また、放電用コンデンサー３９ａの容量は、パルス電流の半値幅が１０〜１００μｓとなるように設定される。
【００３８】
放電用コンデンサー３９ｂとスイッチング素子４０ｂとインダクター４１とレーザー発振制御部５１とにより、レーザー光１１ａの緩和振動を抑制する緩和振動抑制手段が構成される。ここで、スイッチング素子４０ｂは、通常はアノード電流の通過を阻止するが、ゲートに発光指令信号５１ｂが印加されるとアノード電流を通過させる素子である。また、レーザー発振制御部５１は、スイッチング素子４０ａ，４０ｂのゲートに、それぞれ発光指令信号５１ａ，５１ｂを所定のタイミングで印加する回路である。
なお、スイッチング素子４０ａ，４０ｂとしては、サイリスター、ゲートターンオフサイリスター、ＩＧＢＴなどを使用できる。
【００３９】
次に、電源部３０の動作を説明する。
まず、シマー電源３１を動作させる。シマー電源３１がフラッシュランプ２２の両極間に印加した電圧が約５００〜１０００Ｖとなった時点で、トリガーパルス発生器３４がトリガーパルスを出力する。トランス３５で増幅された電圧５ｋＶ以上のトリガーパルスが電極３６よりフラッシュランプ２２に印加されると、トリガーパルスの電圧によりフラッシュランプ２２の両極間の絶縁が破れ、シマー電流が放電される。これにより、フラッシュランプ２２の両極間の電圧は約１００〜２００Ｖに低下する。シマー電流は約１００ｍＡの直流電流であり、この後もフラッシュランプ２２の両極間で放電され続ける。
【００４０】
一方、スイッチング素子４０ａ，４０ｂは通常、ゲートに発光指令信号５１ａ，５１ｂが印加されておらず、阻止状態となっている。この状態で、充電用主電源３７は放電用コンデンサー３９ａ，３９ｂのそれぞれを充電する。そして放電用コンデンサー３９ｂに充電された電圧が約３００Ｖ以上となると、レーザー発振制御部５１はスイッチング素子４０ｂのゲートに発光指令信号５１ｂを印加する。これによりスイッチング素子４０ｂは導通状態となり、放電用コンデンサー３９ｂに充電された電圧はインダクター４１およびフラッシュランプ２２に印加される。このとき、インダクター４１の積分作用により、フラッシュランプ２２に供給される電流Ｉｂの立ち上がりは、図６（Ａ）の点線で示すように緩やかになる。上述したようにフラッシュランプ２２の両極間の電圧は約１００〜２００Ｖに低下しているので、この電流Ｉｂはフラッシュランプ２２の両極間で放電される。
【００４１】
続いて、レーザー発振制御部５１はスイッチング素子４０ａのゲートに発光指令信号５１ａを印加する。これによりスイッチング素子４０ａは導通状態となり、放電用コンデンサー３９ａに充電された電圧はフラッシュランプ２２に印加される。これにより、フラッシュランプ２２の両極間で、図６（Ａ）に実線で示すような立ち上がりが急峻なパルス電流Ｉａが放電される。
このときのフラッシュランプ２２の放電電流（シマー電流を除く）Ｉｃは、図６（Ｂ）に示すように電流Ｉａ，Ｉｂを合成したものとなる。この電流Ｉｃは放電用コンデンサー３９ａからのパルス電流Ｉａよりも立ち上がりが緩やかになっている。
【００４２】
フラッシュランプ２２の放電電流の立ち上がりが急峻であるほど、レーザーヘッド部２０から出力されるレーザー光１１ａの緩和振動が大きくなる。そこで、まず立ち上がりが緩やかな電流Ｉｂをフラッシュランプ２２に印加する。このときレーザー光１１ａに発生する緩和振動は小さい。そして、この電流Ｉｂによる緩和振動がおさまった時点で、パルス電流Ｉａをフラッシュランプ２２に印加するようにする。これによりレーザー光１１ａの緩和振動を極めて小さくできる。
【００４３】
また、電源部３０では、フラッシュランプ２２でシマー電流が常時放電させているが、これによりフラッシュランプ２２でパルス電流Ｉａが放電されるときの放電インピーダンスの変化を小さくできる。これにより、パルス放電の繰り返しが容易に高速化できると共に、発光効率が高まる、フラッシュランプ２２の寿命が延びるといった利点を併せて得られる。
【００４４】
前述したように、大型構造物を非破壊検査するには最高周波数１０〜１００ｋＨｚの振動を励起することが望ましく、そのためにはパルス幅が１０〜１００μｓのレーザー光を照射する必要がある。しかし、従来からあるレーザー超音波法で使用されていたＱスイッチ方式の固体レーザー発振器では、このような長パルスのレーザー光を得られない。
【００４５】
図７は、Ｑスイッチ方式の固体レーザー発振器でＱスイッチを使用せずにノーマル発振させて得られたレーザー光の波形を示す図である。横軸は時間であり、縦軸は光強度である。図７に示したレーザー光の包絡線の半値幅は４０μｓであるが、このレーザー光には緩和振動によるスパイク状のパルスが多数発生している。緩和振動の強度はピークパワーの９０％以上と大きく、また各パルスの半値幅は数ｎｓと極めて短い。このように、従来からレーザー超音波法で使用されていたＱスイッチ方式の固体レーザー発振器を転用したのでは、長パルスのレーザー光を得られない。
【００４６】
図８は、図４および図５に示した構成を有するレーザー光源１１で得られたレーザー光１１ａの波形を示すグラフである。横軸は時間ｔ［μｓ］であり、縦軸は光強度［ａ．ｕ．］である。図８に示したレーザー光１１ａは、半値幅が５０μｓであり、しかも緩和振動の強度がピークパワーの５０％以下に抑制されている。図１に示した非破壊検査装置ではこのようなレーザー光１１ａを使用するので、最高周波数１０〜１００ｋＨｚ程度の低周波振動を効率的に励起できる。
【００４７】
次に、レーザー光源１１の防振構造について説明する。図９は、レーザー光源１１の電源部３０および冷却部５２の外部筐体の側面図である。
レーザー光源１１では、放電用コンデンサー３９ａの高速放電によりフラッシュランプ２２を発光させる。このとき放電用コンデンサー３９ａから強い電磁力が発生する。従来からあるレーザー超音波法で使用されていたレーザー光源１１１ａは床にキャスターなどで設置されており、特別な防振構造を有していなかった。しかし、これではレーザー光源１１１ａが高速放電に伴う電磁力により大きく振動してしまう。この振動は床や空気を介してレーザー干渉計１１３に伝わり、レーザー干渉計１１３の出力信号のノイズとなるという問題があった。
【００４８】
そこで、図１に示した非破壊検査装置のレーザー光源１１では、以下のような防振構造とした。まず、放電用コンデンサー３９ａを収容するケース３９ｃおよび電源部３０の筐体３０ａは従来は鉄などで形成されていたが、このケース３９ｃおよび筐体３０ａを真鍮などの非磁性体で形成した。これにより、高速放電に伴って電磁力が発生しても、ケース３９ｃおよび筐体３０ａは振動しない。
また、図９に示すように、電源部３０の筐体３０ａおよび冷却部５２ａの筐体５２ａを床５５上に配置する際に、防振ゴム５４を介在させている。これにより、筐体３０ａ，５２ａが振動したとしても、その振動は床５５に伝わらない。
レーザー光源１１はこのような防振構造を有しているので、高速放電に伴う電磁力による振動は抑制される。したがって、振動検出器１３で検出された振動波形にのるノイズが小さくなるので、検査精度が高くなる。
【００４９】
次に、図１に示した装置による構造物の非破壊検査に関する実験について説明する。
まず、レーザー光源１１として、図４および図５に示した構成を有する長パルスＹＡＧレーザーのプロトタイプを試作した。このレーザー光源１１は、大容量コンデンサー３９ａの高速放電によるフラッシュランプ発光を用いて、パルス幅５０μｓ、エネルギー２．５Ｊの発振を行うことができた。これと同時に、レーザー光１１ａの広がり角を１０ｍｒａｄ以下に制限できた。
このレーザー光源１１を用いた予備実験で、周波数１〜５０ｋＨｚの振動を効率的に発生させることができ、２１０×１００×３０ｍｍのレンガブロックや１００×１００×５ｍｍのタイルの明瞭な共振スペクトルを非接触で検出できることを初めて確認した。また、レーザー光１１ａはパルス幅が長いので大エネルギーのわりにピークパワーが小さく、部材表面にアブレーション損傷が生じない完全な非破壊検査を行えた。
【００５０】
各実験毎に詳細に説明する。
実験１．従来からあるレーザー超音波法と本発明とのＳ／Ｎ比に関する比較：
試料として２１０×１００×３０ｍｍのレンガブロックを用意した。このレンガブロックの１００×３０ｍｍの面を下にして平面上に置く。このときレンガブロックは平面に接着されておらず、完全剥離の状態となっている。この状態で、レンガブロックの２１０×１００ｍｍの面にパルスレーザー光を照射する。このとき励起された振動に基づく超音波を照射位置から１ｍ遠隔で受信し、周波数分析を行って共振スペクトルを得る。
【００５１】
まず、従来からあるレーザー超音波法で用いられているパルス幅８ｎｓのレーザー光１１１ａを照射した。図１０は、このときのレンガブロックの振動波形を示すグラフであり、図１０（Ａ）にはレーザー干渉計１１３による検出波形が示されており、図１０（Ｂ）にはレーザー干渉計１１３による検出波形をフーリエ変換して得られたパワースペクトルが示されている。図１０（Ａ），（Ｂ）には、レーザー光１１１ａのエネルギーを１１６ｍＪ，８７．８ｍＪ，５７．５ｍＪ，３５．０ｍＪとしたときの波形、およびバックグラウンドの波形が上から順に示されている。
図１０（Ａ）から分かるように、このときのレンガブロックの振動速度ｖの振幅は０．００５ｍｍ／ｓ以下と小さい。また、図１０（Ｂ）では５ｋＨｚ付近に共振スペクトルが見られるが、この共振スペクトルの振幅が小さく、バックグラウンドノイズに埋没している。
【００５２】
次に、上記レーザー光源１１のプロトタイプによるパルス幅５０μｓのレーザー光１１ａを照射した。図１１は、このときのレンガブロックの振動波形を示すグラフであり、図１１（Ａ）には振動検出器１３による検出波形が示されており、図１１（Ｂ）には振動検出器１３による検出波形を計算機１４でフーリエ変換して得られたパワースペクトルが示されている。図１１（Ａ），（Ｂ）には、レーザー光１１ａのエネルギーを２．０８Ｊ（ピークパワー４２ｋＷ），１．２５Ｊ（ピークパワー２５ｋＷ）としたときの波形、およびバックグラウンドの波形（図１１（Ｂ）のみ）が上から順に示されている。
【００５３】
図１１（Ａ）から分かるように、このときのレンガブロックの振動速度ｖの振幅は０．０３〜０．０５ｍｍ／ｓであり、振動速度ｖの振幅が従来の１０倍程度となっている。また、図１１（Ｂ）では５ｋＨｚ付近に明瞭な共振スペクトルが見られる。この共振スペクトルはレンガブロックが剥離状態となっていることを示すものである。したがって、長パルスのレーザー光１１ａを用いることで、検査対象である構造物の状態を高感度で検出できることが分かる。
【００５４】
図１２は、図１１（Ａ）を基に作成されたレーザー光１１ａのピークパワーＩ_０ とレンガブロックの振動速度ｖの振幅との関係を示すグラフである。明瞭な共振スペクトルを得るには、振動検出器１３によって検出された振動速度ｖの振幅は０．０１ｍｍ／ｓ以上であることが望ましい。したがって、検査対象に励起された振動を照射位置から１ｍ遠隔で検出する場合には、ピークパワーＩ_０ が１０ｋＷ以上のレーザー光１１ａを用いればよいことが図１２から分かる。このようなレーザー光１１ａを用いることで、遠隔検査を正確に行える。
【００５５】
実験２．剥離の検出：
図１３は、この実験の試料の構造を説明するための断面図である。３００×３００×６０ｍｍのコンクリート板６１の上に、９８×９８×５ｍｍの磁器タイル（内装用タイル １００角）６２が、１００×１００×３ｍｍの樹脂配合セメント６３によって接着されている。ただし、タイル６２とセメント６３の界面には、３０×３０×０．３ｍｍのナイロンシート６４があらかじめ挟み込まれており、人工剥離が形成されている。ここではナイロンシート６４が挟み込まれている部分を剥離部、ナイロンシート６４が挟み込まれていない部分を健全部と呼ぶ。
【００５６】
まず、健全部のタイル６２表面に、上記レーザー光源１１のプロトタイプによるパルス幅５０μｓのレーザー光１１ａを照射した。図１４は、このときの健全部の振動波形を示すグラフであり、図１４（Ａ）には振動検出器１３による検出波形が示されており、図１４（Ｂ）には振動検出器１３による検出波形を計算機１４でフーリエ変換して得られたパワースペクトルが示されている。
続いて、剥離部のタイル６２表面に同じくレーザー光１１ａを照射した。図１５は、このときの剥離部の振動波形を示すグラフであり、図１５（Ａ）には振動検出器１３による検出波形が示されており、図１５（Ｂ）には振動検出器１３による検出波形を計算機１４でフーリエ変換して得られたパワースペクトルが示されている。
【００５７】
図１４（Ｂ）と図１５（Ｂ）とを対比すると、図１５（Ｂ）には１５ｋＨｚ付近に強い共振スペクトルが現れていることが分かる。この共振スペクトルは、剥離と表面との間のタイル部分の熱膨張によって誘起された振動に基づくものである。この共振スペクトルの有無により、剥離の有無を知ることができる。
【００５８】
次に、ナイロンシート３４による人工剥離の大きさが異なる５種類の試料を用意した。表１は、人工剥離の大きさと、人工剥離の面積率Ｒａを示す表である。ここで、人工剥離の面積率Ｒａは、（人工剥離の面積）／（タイル６２の面積）により求められたものである。
【００５９】
【表１】

【００６０】
そして、各試料の剥離部のタイル６２表面に、上記レーザー光源１１のプロトタイプによるパルス幅５０μｓのレーザー光１１ａを順次照射した。図１６は、このときの各試料の剥離部の振動波形をフーリエ変換して得られたパワースペクトルを示すグラフである。このグラフには、人工剥離が小さい試料のパワースペクトルが下から順に示されている。このグラフの横軸は周波数ｆ［ｋＨｚ］、縦軸は強度である。ただし、人工剥離の大きさが５０×５０ｍｍ，６９．３×６９．３ｍｍ，８４．９×８４．９ｍｍの各試料のパワースペクトルの強度は、それぞれ１／１００，１／２００，１／４００に圧縮されている。
【００６１】
各パワースペクトルにおいて、矢印を付した共振スペクトルが、剥離と表面との間のタイル部分に誘起された振動に基づくものである。剥離が大きいほど、その剥離に基づく共振周波数が小さくなることが分かる。
図１７は、各試料の人工剥離の面積率Ｒａ［％］と、その人工剥離に基づく共振周波数ｆ［ｋＨｚ］との関係を示すグラフである。このグラフを基に、得られた共振周波数から剥離の面積率を推定できる。
この実験２はタイル６２とコンクリート板６１との間に人工的に剥離を設けた試料を用いた実験であるが、大型構造物の表面にほぼ平行な剥離状の亀裂も同様に検出でき、また共振周波数から亀裂の面積率も推定できる。
【００６２】
実験３．厚さの検出：
３００×３００×６０ｍｍおよび２５０×２５０×５０ｍｍの２個のコンクリート板を用意する。これらのコンクリート板の上に１００×１００×３ｍｍの樹脂配合セメントで９８×９８×５ｍｍの磁器タイル（内装用タイル １００角）を接着させて、試料（イ），（ロ）を作成した。ただし、いずれの試料（イ），（ロ）においても、タイルとコンクリート板の間に剥離は存在しない。
これらの試料（イ），（ロ）のタイル表面に、上記レーザー光源１１のプロトタイプによるパルス幅５０μｓのレーザー光１１ａを照射した。図１８は、このときの試料（イ），（ロ）の振動波形を示すグラフであり、図１８（Ａ），（Ｂ）にはそれぞれ試料（イ），（ロ）の振動波形が示されており、図１８（Ｃ），（Ｄ）にはそれぞれ試料（イ），（ロ）の振動波形をフーリエ変換して得られたパワースペクトルが示されている。
【００６３】
図１８（Ｃ）に示されるように、試料（イ）の熱膨張によって誘起された振動の共振周波数は、５．３ｋＨｚおよび６．０ｋＨｚである。また、図１８（Ｄ）に示されるように、試料（ロ）の熱膨張によって誘起された振動の共振周波数は、６．２ｋＨｚおよび７．５ｋＨｚである。この共振周波数の違いは、試料（イ），（ロ）のコンクリート板の厚さの違いによるものである。したがって、得られた共振周波数からコンクリート板の厚さを推定できる。
この技術は大型構造物に対しても応用できる。例えば、同じ方法でトンネルの壁の厚さなどを推定できる。
【００６４】
以上の実験１〜３の実験結果から明らかなように、パルス幅が１０〜１００μｓであり緩和振動が抑制されたレーザー光１１ａを用いることで、大型構造物に対しても、励起された振動を高感度で検出できる。また、レーザー光１１ａのピークパワーＩ_０ を１０ｋＷとすることで、照射位置から１ｍ遠隔でも励起された振動を高感度で検出できる。したがって、図１に示した装置により、大型構造物の健全性を遠隔から高感度で非破壊検査できる。しかも、図１に示した装置による非破壊検査は、人手によらず、しかも非接触で行えるので、自動化が容易である。
なお、図１に示した非破壊検査装置は、トンネルに限らず、交通機関、発電施設、居住用建築などの大型構造物の非破壊検査に広範囲に適用できる。
【００６５】
また、例えば壁の内部が奥行き方向に２層構造となっている場合には、図１に示した装置により、両層の界面の接着が不完全である部分を検出できる。このような部分に長パルスのレーザー光１２ａを照射すると、表層には熱膨張により応力が発生して両層の界面が剥離し、上述したのと同様の原理で照射領域に振動が励起されるからである。これは、柱の内部に鉄筋が埋設されており、柱のコンクリートと鉄筋との接着が不完全である場合も同様である。
また、２層構造を有する大型構造物の表層の厚さが１ｃｍ程度である場合には、両層の界面が完全に密着していても、表層の厚さを正確に計測できる。１Ｊ程度の長パルスレーザー光１２ａを照射して表層の温度が数１０℃程度上昇すると、両層の界面の接着が一時的に弱まり、両層の界面が剥離したときと同様に表層の照射領域が振動するからである。
【００６６】
以上のように図１に示した非破壊検査装置により、大型構造物の内部構造として、構造物の表面とほぼ平行にできた１〜５０ｃｍ程度の亀裂、１０〜１００ｃｍ程度の構造物の厚さ、多層構造を有する構造物あるいは他の部材が埋設された構造物における界面の接着が不完全である部分、１ｃｍ程度の構造物の表層の厚さなどを計測できる。
また、図１に示した非破壊検査装置は、数ｍ以上のコンクリート劣化、亀裂などの有無とその形状を迅速かつ精密に計測できる移動型検査システムにも応用できる。これは現在各方面で問題となっている大型構造物の安全性の飛躍的向上に貢献すると同時に、新しい検査産業の創出にもつながると考えられる。
【００６７】
【発明の効果】
以上説明したように、本発明では、検査に用いるパルス光のパルス幅を１０〜１００μｓとすると共に、緩和振動をピークパワーの５０％以下に抑制する。これにより大型構造物での減衰が小さい１０〜１００ｋＨｚ程度の振動を効率的に励起できるので、大型構造物の健全性を高感度で計測できる。また、本発明では、検査に用いるパルス光のピークパワーを１０ｋＷ以上とする。これにより、大型構造物の健全性を遠隔計測できる。
また、本発明では、光源をランプ励起のノーマル発振固体レーザー発振器で構成する。また、ランプ励起用の電源部に予備放電回路を設けて、励起ランプを常時放電状態にしておく。これにより、パルス電流が励起ランプで放電されるときの放電インピーダンスの変化を小さくできるので、パルス放電の繰り返しが容易に高速化できると共に、発光効率が高まる、励起ランプの寿命が延びるといった効果を得られる。
【００６８】
また、本発明では、光源の高速放電回路のケース、光源の電源部の筐体を、非磁性体で形成する。また、光源の電源部および冷却部の筐体を、防振構造とする。これにより、高速放電に伴う電磁力による振動は抑制される。したがって、振動検出手段で検出された振動波形にのるノイズが小さくなるので、検査精度が高くなる。
また、本発明では、パルス光の広がり角を１０ｍｒａｄ以下に制限する。これによりパルス光の指向性が高くなるので、所望の領域のみの検査データを取得できる。
また、本発明では、パルス光の照射位置を走査することにより、大面積の検査を容易に行える。ここで、走査手段として音響光学的変調器を用いることにより、高速走査が可能となる。
【図面の簡単な説明】
【図１】本発明による構造物の非破壊検査装置の一実施の形態の構成を示すブロック図である。
【図２】超音波の発生原理の説明図である。
【図３】物体表層の温度分布を示すグラフである。
【図４】レーザー光源の構成図である。
【図５】レーザー光源の電源部の構成を示す回路図である。
【図６】フラッシュランプに供給される電流（シマー電流を除く）を示すグラフである。
【図７】Ｑスイッチ方式の固体レーザー発振器でＱスイッチを使用せずにノーマル発振させて得られたレーザー光の波形を示す図である。
【図８】図４および図５に示した構成を有するレーザー光源１１で得られたレーザー光の波形を示すグラフである。
【図９】レーザー光源の電源部および冷却部の外部筐体の側面図である。
【図１０】パルス幅８ｎｓのレーザー光を照射したときのレンガブロックの振動波形を示すグラフである。
【図１１】パルス幅５０μｓのレーザー光を照射したときのレンガブロックの振動波形を示すグラフである。
【図１２】レーザー光のピークパワーとレンガブロックの振動速度の振幅との関係を示すグラフである。
【図１３】実験２の試料の構造を説明するための断面図である。
【図１４】パルス幅５０μｓのレーザー光を照射したときの健全部の振動波形を示すグラフである。
【図１５】パルス幅５０μｓのレーザー光を照射したときの剥離部の振動波形を示すグラフである。
【図１６】パルス幅５０μｓのレーザー光を照射したときの各試料の剥離部の振動波形に基づくパワースペクトルを示すグラフである。
【図１７】各試料の人工剥離の面積率と、その人工剥離に基づく共振周波数との関係を示すグラフである。
【図１８】パルス幅５０μｓのレーザー光を照射したときの試料（イ），（ロ）の振動波形を示すグラフである。
【図１９】従来から行われているレーザー超音波法により物体内部の欠陥を検出するための検査装置の一構成例を示すブロック図である。
【図２０】従来から行われているレーザー超音波法で用いられているレーザー光の波形を示す図である。
【符号の説明】
１…トンネル、２…トンネルの表面、３…亀裂、４…板状部分、４ａ…超音波、１０…構造物の非破壊検査装置、１１…レーザー光源、１１ａ，１２ａ…レーザー光、１２…音響光学的変調器、１３…振動検出器、１３ａ，１４ａ…データ、１４…計算機、１４ｓ，１４ｔ…制御信号、１５…表示装置、２０…レーザーヘッド部、２１…レーザーロッド、２２…フラッシュランプ、２３…集光器、２４ａ…全反射鏡、２４ｂ…出力鏡、２５…エキスパンダー、２５ａ…凸レンズ、２５ｂ…凹レンズ、２６…ポラライザー、３０…電源部、３０ａ，５２ａ…筐体、３１…シマー電源、３２…抵抗、３３…ダイオード、３４…トリガーパルス発生器、３５…トランス、３６…電極、３７…充電用主電源、３８ａ，３８ｂ，４１…インダクター、３９ａ，３９ｂ…放電用コンデンサー、３９ｃ…ケース、４０ａ，４０ｂ…スイッチング素子、５１…レーザー発振制御部、５１ａ，５１ｂ…発光指令信号、５２…冷却部、５４…防振ゴム、５５…床、６１…コンクリート板、６２…磁気タイル、６３…樹脂配合セメント、６４…ナイロンシート。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nondestructive inspection method and apparatus for inspecting the internal structure of a large structure nondestructively, and more particularly to a nondestructive inspection method and apparatus capable of inspecting a structure without contact.
[0002]
[Prior art]
In large structures that require high reliability, such as transportation, power generation facilities, and residential buildings, accidents due to deterioration over time and poor construction frequently occur. The cause is often peeling of the surface of the structure or cracking inside the structure that can be easily found if nondestructive inspection is performed. Therefore, it is desired to periodically inspect for peeling and cracking of large structures, but a simple inspection method that enables rapid automatic inspection of a large area has not been developed.
[0003]
Conventionally, for inspection of peeling and cracking of a large structure, a hitting test in which the surface is hit with a hammer and the vibration response is measured with an ear or a meter has been mainly used. However, since the hammering inspection must be relied on manually, it is difficult to perform a large-area rapid automatic inspection by this method. In addition, although automation is also examined for the ultrasonic flaw detection method and the X-ray transmission method, since it is necessary to bring the sensor close to the structure, it is difficult to automate the surface with many irregularities. On the other hand, the inspection method using electromagnetic waves can perform non-contact measurement, but the sensor cannot be separated from the structure by several cm or more. Moreover, although a cavity with a thickness of several centimeters or more has sensitivity, it cannot be applied to a thinner peeling or crack.
[0004]
Under such circumstances, proposals have been made to use a laser ultrasonic method for the inspection of peeling and cracking of large structures. FIG. 19 is a block diagram showing a configuration example of an inspection apparatus for detecting defects inside an object by a conventional laser ultrasonic method. In this laser ultrasonic method, first, the surface of the object 101 is irradiated with pulsed laser light 111 a generated by the laser light source 111. In the region irradiated with the laser beam 111a, vibration is excited by rapid thermal expansion, and an ultrasonic wave 104a based on this vibration is generated. The ultrasonic wave 104a is detected by the non-contact laser interferometer 113, and the output signal 113a of the laser interferometer 113 is arithmetically processed by the arithmetic processing unit 114, thereby measuring the minute defect 103 inside the object 101.
[0005]
However, this laser ultrasonic method is a technique developed for the purpose of precision inspection such as quality inspection of precision machine parts. Therefore, it is desired to have high sensitivity to the thickness (about 1 cm) and minute defects (about 0.01 to 1 mm) 103 of inspection objects such as metals, composite materials, and fine ceramics. Therefore, the ultrasonic wave 104a of 10 MHz to 1 GHz is generated in the thermoelastic mode using the short-pulse laser beam 111a having a half width (pulse width) of 1 to 100 ns (nanosecond) as shown in FIG. ing. Further, as the laser light source 111, a Q-switch type solid laser oscillator capable of outputting the laser light 111a having the above-described half-value width is used.
[0006]
If this laser ultrasonic method is used, it is possible to automatically measure defects inside the structure from a remote location. However, since the frequency of the ultrasonic wave 104a is as high as 10 MHz to 1 GHz as described above, the ultrasonic wave 104a is greatly attenuated in large metal members, concrete, refractory bricks, and the like, and the detection sensitivity of the ultrasonic wave 104a is low. In order to prevent this decrease in sensitivity, it is necessary to use low frequency ultrasonic waves, and the optimum ultrasonic frequency is 1 to 100 kHz. Even with the conventional laser ultrasonic method, it is possible to measure vibration in this frequency band, but the measurement sensitivity is still low because of low vibration excitation efficiency.
[0007]
[Problems to be solved by the invention]
Thus, conventionally, a non-destructive inspection method that is easy to automate and can measure the soundness of a large structure from a remote location with high sensitivity has not been developed.
The present invention has been made under such circumstances, and an object thereof is to realize an automated nondestructive inspection method capable of remotely measuring the soundness of a large structure.
Another object is to realize a nondestructive inspection method capable of measuring the soundness of a large structure with high sensitivity.
[0008]
[Means for Solving the Problems]
In order to achieve such an object, the nondestructive inspection method for a structure according to the present invention has a pulse width of 10 microseconds to 100 microseconds, a peak power of 10 kW or more, and relaxation vibrations are suppressed. Generate pulsed light, and use this pulsed light to display the structure.On the faceA first step of irradiating, a second step of detecting vibration generated by absorption of light energy below and on the surface of the structure in the region irradiated with pulsed light, and a structure based on the detected vibration waveform A third step of generating information relating to the internal structure of the objectThe pulsed light is a laser beam generated by a preliminary discharge circuit that constantly supplies a predetermined direct current to the excitation lamp and a high-speed discharge circuit that supplies a pulse current to the excitation lamp. The preliminary discharge circuit and the high-speed discharge circuit are Stored in a housing made of non-magnetic material or a housing having an anti-vibration structureIs characterized by The maximum frequency of vibration excited in the thermoelastic mode by the irradiation of the pulsed light is substantially proportional to the reciprocal of the pulse width of the irradiated pulsed light. Therefore, by irradiating pulsed light having a pulse width of 10 to 100 μs (microseconds), vibration having a maximum frequency of about 10 to 100 KHz can be efficiently excited. The vibration in this frequency band can be detected with high sensitivity since the attenuation of a large metal member is small.Further, by housing the preliminary discharge circuit and the high-speed discharge circuit in the above-described case, the case does not vibrate even when electromagnetic force is generated due to high-speed discharge.
[0009]
In the first step of the present invention, the relaxation oscillation of the pulsed light may be suppressed to 50% or less of the peak power.
In the first step, the surface of the structure may be irradiated with pulsed light without contact in the first step, and the vibration may be detected without contact with the surface of the structure in the second step.
In the third step of the present invention, in the third step, the detected vibration waveform is subjected to Fourier transform to obtain a resonance frequency of the vibration, and at least one of the area of the gap formed in the structure and the thickness of the structure is subjected to vibration. Estimated from the resonance frequency.
In the first step, the present invention scans the irradiation position of the pulsed light. Thereby, the vibration distribution over a wide range of the structure can be measured.
[0010]
In addition, the nondestructive inspection apparatus for a structure according to the present invention is generated by a light source that irradiates the surface of the structure with pulsed light and absorption of light energy below and on the surface of the structure in the region irradiated with the pulsed light. Vibration detection means for detecting vibration, and arithmetic processing means connected to the output side of the vibration detection means and generating information on the internal structure of the structure based on the vibration waveform detected by the vibration detection means, The light source has relaxation vibration suppressing means for suppressing relaxation vibration of the pulsed light, the pulse width of the pulsed light is not less than 10 microseconds and not more than 100 microseconds, and the peak power of the pulsed light is not less than 10 kWFurthermore, the light source includes a laser oscillator of a normal oscillation solid laser that obtains a laser output by exciting the laser medium with light from the excitation lamp, a preliminary discharge circuit that constantly supplies a predetermined direct current to the excitation lamp, and an excitation lamp. A power supply unit including a high-speed discharge circuit for supplying a pulse current, and at least one of a case of the high-speed discharge circuit and a housing of the power supply unit is formed of a non-magnetic material.Is characterized byBy keeping the excitation lamp in a constant discharge state by the preliminary discharge circuit, the change in discharge impedance when the pulse current from the fast discharge circuit is discharged by the excitation lamp can be reduced. Further, by forming at least one of the case of the high-speed discharge circuit and the case of the power supply unit with a non-magnetic material, the case and the case do not vibrate even when electromagnetic force is generated due to high-speed discharge.
[0011]
In addition, the nondestructive inspection apparatus for a structure according to the present invention is generated by a light source that irradiates the surface of the structure with pulsed light and absorption of light energy below and on the surface of the structure in the region irradiated with the pulsed light. Vibration detection means for detecting vibration, and arithmetic processing means connected to the output side of the vibration detection means and generating information on the internal structure of the structure based on the vibration waveform detected by the vibration detection means, The light source has relaxation vibration suppressing means for suppressing relaxation vibration of the pulsed light, the pulse width of the pulsed light is not less than 10 microseconds and not more than 100 microseconds, the peak power of the pulsed light is not less than 10 kW, ,light sourceThe laser oscillator of a normal oscillation solid-state laser that obtains laser output by exciting the laser medium with light from the excitation lampWhen,Pre-discharge circuit that constantly supplies a predetermined direct current to the excitation lampAnd encouragementPower supply unit comprising a high-speed discharge circuit for supplying a pulse current to a starting lampAnd a cooling part for cooling the laser medium and the excitation lamp, and the casing of the power supply part and the cooling part is characterized by having a vibration-proof structure..
[0012]
Here, the relaxation vibration suppressing means of the light source may be a means for suppressing the relaxation vibration of the pulsed light to 50% or less of the peak power of the pulsed light. Further, the relaxation oscillation suppressing means may be a means for moderating the rise of the pulse current from the high-speed discharge circuit.
The light source may be a means for irradiating the surface of the structure with non-contact pulsed light, and the vibration detection means may be a means for detecting vibration without contact with the surface of the structure.
[0013]
In the nondestructive inspection apparatus for a structure, the light source may have a spread angle control unit that limits a spread angle of the pulsed light to 10 mrad or less. This increases the directivity of the pulsed light.
The nondestructive inspection apparatus for a structure may include a scanning unit that is disposed on the output side of the light source and scans the irradiation position of the pulsed light.
When the pulsed light is linearly polarized light, the scanning means may be constituted by an acousto-optic modulator. By using an acousto-optic modulator, scanning can be performed at high speed. In addition, since the pulse power of a long pulse having a pulse width of 10 to 100 μs has a peak power smaller than that of the laser light by the Q switch method, the surface of the acousto-optic modulator is hardly damaged by ablation.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing the configuration of an embodiment of a nondestructive inspection apparatus for structures according to the present invention. In this figure, the tunnel 1 is illustrated as a structure to be inspected. Further, it is assumed that a crack (gap) 3 substantially parallel to the surface (wall surface) 2 of the tunnel 1 is formed inside the tunnel 1.
The nondestructive inspection apparatus 10 includes a laser light source 11, an acousto-optic modulator (scanning means) 12, a vibration detector 13, a calculator 14, and a display device 15.
[0015]
The laser light source 11 is a laser oscillator of a normal oscillation solid laser that outputs a long pulse laser beam 11a. The laser beam 11a has a half width (hereinafter sometimes referred to as pulse width) of 10 μs or more and 100 μs or less, and a peak power of 10 kW or more. Further, the relaxation vibration of the laser beam 11a is suppressed to 50% or less of the peak power of the laser beam 11a.
The acousto-optic modulator 12 is disposed on the output side of the laser light source 11 and is spaced from the surface 2 of the tunnel 1 by about 1 m. The acousto-optic modulator 12 functions as scanning means for scanning the irradiation position on the tunnel surface 2 by deflecting the laser light 11a.
[0016]
The vibration detector 13 is also arranged at a distance of about 1 m from the surface 2 of the tunnel 1. In the region irradiated with the laser light 12a from the acousto-optic modulator 12, vibration is excited, and an ultrasonic wave 4a having the same waveform as this vibration is generated. The vibration detector 13 detects the vibration by directly detecting the vibration or by receiving the ultrasonic wave 4a. As the vibration detector 13, a laser interferometer or an ultrasonic detection microphone can be used.
[0017]
The calculator 14 is connected to the output side of the vibration detector 13, and is an arithmetic processing unit that performs frequency analysis on the vibration waveform data 13 a detected by the vibration detector 13 and generates information on the internal structure of the tunnel 1. It has the function of The calculator 14 also has a function as control means for controlling the operations of the laser light source 11, the acousto-optic modulator 12, the vibration detector 13, and the display device 15. In particular, the control signals 14s and 14t are output to the laser light source 11 and the acousto-optic modulator 12, respectively, to synchronize the operations of the laser light source 11 and the acousto-optic modulator 12.
The display device 15 is connected to the output sides of both the vibration detector 13 and the computer 14 and displays data 13a and 14a output from the vibration detector 13 and the computer 14, respectively.
[0018]
Next, the principle that the ultrasonic wave 4a is generated in the thermoelastic mode by the irradiation of the pulse laser beam 12a will be described.
FIG. 2 is an explanatory diagram of the principle of generation of the ultrasonic wave 4a, and schematically shows a cross section of the tunnel 1 shown in FIG. Here, it is assumed that the thickness of the plate-like portion 4 between the surface 2 and the crack 3 is sufficiently thin.
[0019]
FIG. 2A shows a state before the laser beam 12a is irradiated. When the surface 2 is irradiated with the laser beam 12a, the temperature of the surface 2 and the plate-like portion 4 rises due to light energy absorption of the laser beam 12a, and the plate-like portion 4 suddenly changes as shown in FIG. Thermal expansion. When irradiation with the laser beam 12a is stopped in this state, the plate-like portion 4 is cooled and contracts rapidly. At this time, the plate-like portion 4 contracts excessively as shown in FIG. Thereafter, the plate-like portion 4 starts to expand again as shown in FIG. Thereafter, the vibration gradually converges while repeating this. In this way, the bending vibration of the plate-like portion 4 is excited. In addition, various vibration modes such as thickness vibration and torsional vibration are excited by the shape of the member corresponding to the plate-like portion 4. The ultrasonic wave 4a is generated with the vibration excited in this way.
[0020]
The maximum frequency of vibration excited in the thermoelastic mode by irradiation with the laser beam 12a is substantially proportional to the reciprocal of the pulse width of the laser beam 12a. Therefore, by irradiating the laser beam 12a with a long pulse having a pulse width of 10 to 100 μs, vibrations with a maximum frequency of about 10 to 100 kHz can be excited efficiently. The vibration in this frequency band has a small attenuation even in a large metal member having a thickness of about 10 to 100 cm, concrete and refractory bricks. Therefore, the vibration detector 13 can detect the vibration of the plate-like portion 4 with high sensitivity.
Note that if the thickness of the plate-like portion 4 becomes larger than the wavelength of the elastic wave having a frequency converted from the pulse width of the laser beam 12a, the vibration of the plate-like portion 4 is not excited. However, when the laser beam 12a having the above-described pulse width is used, since the frequency is low and the wavelength is long, even a structure having a normal size has a size of less than the wavelength and vibration occurs.
[0021]
Generation of vibration due to laser light occurs due to a temperature rise in the irradiation region. For this reason, the distribution of temperature rise in the depth direction is important. The temperature distribution is determined by the thermal conductivity κ and the light absorption coefficient γ, which are strongly dependent on the substance. In particular, the difference between a metal and an insulator is large.
In metals, it can be assumed that light absorption occurs on the surface of the object and that the temperature rise inside the object is caused by heat conduction. Further, it is assumed that the laser beam is sufficiently thick and uniform in the direction of the object surface, and the light absorption power density (peak power per unit area of the laser light) is time 0 <t <t.₀  (T₀  Is the laser pulse width)₀  (Constant value), 0 otherwise. At this time, after the laser emission ends (t₀  Temperature distribution T (x in the object at <t)₃, T) is given by: X₃  Is the distance from the surface inside the object.
[0022]
[Expression 1]

[0023]
Here, κ is the thermal conductivity, D = κ / (ρC) is the thermal diffusivity, ρ is the density, and C is the heat capacity. However, during laser emission (0 <t <t₀  ) Takes only the first term of equation (1).
On the other hand, in an insulator having a light transmission distance of several tens of μm or more, the thermal diffusion distance in the time of the pulse width of the laser light is relatively small and can be ignored. In this case, the temperature rises in proportion to the irradiation time of the laser beam, and the temperature distribution T (x₃, T) is given by:
[0024]
[Expression 2]

[0025]
Subsequently, the temperature distribution of the object surface layer when irradiated with pulsed laser light is compared between the conventional laser ultrasonic method and the present invention. FIG. 3 is a graph showing the temperature distribution of the object surface layer obtained from the equations (1) to (3) under the following conditions. The horizontal axis of this graph is the distance x from the object surface₃  [Μm], and the vertical axis represents the temperature increase T [K] due to laser irradiation.
[0026]
First, in examining the case of the conventional method, the pulse width t₀  The temperature distribution in the case of irradiating a laser pulse 111a with a short pulse of 10 ns is examined with both a metal and an insulator.
When the object is a metal, the thermophysical constant is assumed to be mild steel, the thermal conductivity κ = 50 W / (m · K), the thermal diffusivity D = 1.3 × 10^-5m²  / S. However, the light absorption coefficient γ is infinite. Also, the absorbed power density I₀  Assumes 20 MW / cm assuming laser light oscillated by the Q switch method²  (Corresponding to a beam diameter of 8 mm and an absorbed energy of 100 mJ). The solid line a in FIG.₀  This is a temperature distribution when a time t = 100 ns, which is 10 times as long, elapses. The temperature rise T is 264 K on the surface, but is as small as 10 K inside 4 μm. In the case of normal temperature (293K), the surface temperature immediately after irradiation exceeds the melting point (1810K), so ablation occurs.
[0027]
When the object is an insulator, the thermal conductivity κ and the thermal diffusivity D are the same values as in the case of metal, and the light absorption coefficient γ = 1 × 10.⁴  [1 / m]. Also, the absorbed power density I₀  200 MW / cm, 10 times that of metal²  And A dotted line b in FIG. 3 is a temperature distribution at time t = 10 ns when the laser emission is finished. The temperature rise T on the outermost surface is 52K, and no ablation occurs, and the temperature rises by 20K or more even within 100 μm.
[0028]
Next, in examining the case according to the present invention, the pulse width t₀  = Temperature distribution when irradiating laser light 11a with a long pulse of 50 μs is examined with both metal and insulator. Absorbed power density I₀  Is 1 MW / cm²  And
If the object is a metal, the pulse width t₀  After the elapse of time t = 500 μs, as shown by the broken line c in FIG. 3, the temperature rise T reached 60K on the surface layer having a thickness of 100 μm. Further, in the case of an insulator through which light is transmitted, due to light transmission and internal absorption, the temperature rises within 100 μm at the time t = 50 μs when the laser emission is finished, as indicated by a dashed line d in FIG. T became 50K. In either case, the temperature rise T on the surface is 600K or less, and no ablation occurs.
[0029]
As can be seen from the above, when the long-pulse laser beam 11a is used, an appropriate temperature rise occurs on a relatively thick surface layer, whether it is a metal or an insulator. Therefore, since vibration due to thermal expansion can be excited in a relatively thick surface layer, it can be seen that defects such as cracks formed in the large structure can be inspected.
On the other hand, the laser beam 11a has a pulse width t.₀  Because of the long peak power I instead of large energy₀  Is small and ablation does not occur on the object surface. For this reason, a complete non-destructive inspection without ablation damage on the surface to be inspected can be performed.
[0030]
Further, the absorption power density I of the laser beam 11a₀  Is 1 MW / cm²  This is below the light resistance limit of the acousto-optic modulator 12 shown in FIG. Accordingly, the surface of the acousto-optic modulator 12 can be used without causing ablation damage. By using the acousto-optic modulator 12, the waveform of the laser beam 11a can be freely controlled. In particular, the acousto-optic modulator 12 optically deflects the laser light 11a instead of mechanically, so that it can be scanned at high speed. Needless to say, a reflection mirror can be used as the scanning means.
[0031]
Next, the laser light source 11 that outputs a long-pulse laser beam 11a will be further described.
FIG. 4 is a configuration diagram of the laser light source 11. The laser light source 11 is a solid laser, and includes a laser head unit 20, a power supply unit 30, a laser oscillation control unit 51, and a cooling unit 52.
Further, the laser head unit 20 is a normal oscillation solid-state laser oscillator, and includes a laser rod 21, a flash lamp 22, a condenser 23 that houses the laser rod 21 and the flash lamp 22, and total reflection that constitutes an optical resonator. The mirror 24a and the output mirror 24b, the expander (expansion angle control means) 25 disposed between one mirror surface of the condenser 23 and the total reflection mirror 24a, the other mirror surface of the condenser 23 and the output mirror 24b And a polarizer 26 disposed between the two.
[0032]
The laser rod 21 is made of a solid laser medium such as Nd: YAG crystal, Er: YAG crystal, Nd: glass crystal, or Nd: YLF crystal. The flash lamp 22 is an excitation lamp that excites the laser rod 21 by irradiating light, and a xenon flash lamp or the like is used. The condenser 23 condenses the light from the flash lamp 22 on the laser rod 21 efficiently.
[0033]
The expander 25 includes a convex lens 25a and a concave lens 25b. Then, by adjusting the refractive index of each lens 25a, 25b and the distance between each lens 25a, 25b, the spread angle of the laser beam 11a output from the laser head unit 20 (ie, the laser light source 11) can be controlled. . Here, in order to increase directivity, the spread angle of the laser beam 11a is limited to 10 mrad or less.
The polarizer 26 is a polarizing element for making the laser beam 11a linearly polarized light. By using the polarizer 26 to obtain a linearly polarized light output, the acoustooptic modulator 12 shown in FIG. 1 enables high-efficiency modulation and deflection. If the acoustooptic modulator 12 is not used, the polarizer 26 may be omitted.
[0034]
The power supply unit 30 is a power supply that supplies a discharge current to the flash lamp 22 of the laser head unit 20. The laser oscillation control unit 51 is connected to the power supply unit 30 and controls the timing at which the power supply unit 30 supplies the discharge current to the flash lamp 22. The cooling unit 52 cools (for example, water-cools) the laser rod 21, the flash lamp 22, and the condenser 23 of the laser head unit 20.
[0035]
FIG. 5 is a circuit diagram showing a configuration of power supply unit 30 shown in FIG.
The anode of the flash lamp 22 is connected to the + terminal of the simmer power supply 31 via the diode 33 and the resistor 32, and the cathode is connected to the − terminal of the simmer power supply 31. The anode of the flash lamp 22 is connected to one end of the discharging capacitor 39a via the switching element 40a, and the cathode is connected to the other end of the discharging capacitor 39a. Further, the anode of the flash lamp 22 is connected to one end of the discharging capacitor 39b via the inductor 41 and the switching element 40b, and the cathode is connected to the other end of the discharging capacitor 39b.
[0036]
The discharging capacitors 39a and 39b are accommodated in a case 39c. Each gate of the switching elements 40 a and 40 b is connected to the laser oscillation control unit 51.
One end of each of the discharging capacitors 39a and 39b is connected to the + terminal of the charging main power supply 37 via the inductors 38a and 38b, and the other end is connected to the − terminal of the charging main power supply 37. Yes.
Further, the power supply unit 30 includes a trigger pulse generator 34 that applies a trigger pulse from the electrode 36 to the flash lamp 22 via the trigger transformer 35.
[0037]
The simmer power supply 31 constitutes a preliminary discharge circuit that constantly supplies a predetermined direct current to the flash lamp 22. The voltage between both terminals (+, −) of the simmer power supply 31 is set lower than the voltage between both terminals (+, −) of the charging main power supply 37.
The discharge capacitor 39a and the switching element 40a constitute a high-speed discharge circuit that supplies a pulse current to the flash lamp 22. Here, the switching element 40a is an element that normally blocks the passage of the anode current, but allows the anode current to pass when the light emission command signal 51a is applied to the gate. The capacity of the discharging capacitor 39a is set so that the half-value width of the pulse current is 10 to 100 μs.
[0038]
The discharging capacitor 39b, the switching element 40b, the inductor 41, and the laser oscillation control unit 51 constitute relaxation vibration suppressing means for suppressing relaxation vibration of the laser light 11a. Here, the switching element 40b is an element that normally blocks the passage of the anode current but allows the anode current to pass when the light emission command signal 51b is applied to the gate. The laser oscillation control unit 51 is a circuit that applies light emission command signals 51a and 51b to the gates of the switching elements 40a and 40b, respectively, at a predetermined timing.
As the switching elements 40a and 40b, thyristors, gate turn-off thyristors, IGBTs, or the like can be used.
[0039]
Next, the operation of the power supply unit 30 will be described.
First, the simmer power supply 31 is operated. The trigger pulse generator 34 outputs a trigger pulse when the voltage applied between the two poles of the flash lamp 22 by the simmer power supply 31 reaches about 500 to 1000V. When a trigger pulse with a voltage of 5 kV or more amplified by the transformer 35 is applied to the flash lamp 22 from the electrode 36, the insulation between the two electrodes of the flash lamp 22 is broken by the voltage of the trigger pulse, and the simmer current is discharged. As a result, the voltage between both electrodes of the flash lamp 22 decreases to about 100 to 200V. The simmer current is a direct current of about 100 mA and continues to be discharged between both poles of the flash lamp 22 thereafter.
[0040]
On the other hand, the light emitting command signals 51a and 51b are not normally applied to the gates of the switching elements 40a and 40b and are in a blocking state. In this state, the charging main power source 37 charges each of the discharging capacitors 39a and 39b. When the voltage charged in the discharging capacitor 39b becomes about 300 V or more, the laser oscillation control unit 51 applies the light emission command signal 51b to the gate of the switching element 40b. As a result, the switching element 40 b becomes conductive, and the voltage charged in the discharging capacitor 39 b is applied to the inductor 41 and the flash lamp 22. At this time, due to the integral action of the inductor 41, the rise of the current Ib supplied to the flash lamp 22 becomes gentle as shown by the dotted line in FIG. As described above, since the voltage between both electrodes of the flash lamp 22 is reduced to about 100 to 200 V, the current Ib is discharged between both electrodes of the flash lamp 22.
[0041]
Subsequently, the laser oscillation control unit 51 applies a light emission command signal 51a to the gate of the switching element 40a. As a result, the switching element 40a becomes conductive, and the voltage charged in the discharging capacitor 39a is applied to the flash lamp 22. As a result, a pulse current Ia having a steep rise as shown by a solid line in FIG. 6A is discharged between both electrodes of the flash lamp 22.
At this time, the discharge current (excluding the simmer current) Ic of the flash lamp 22 is obtained by combining the currents Ia and Ib as shown in FIG. The current Ic rises more slowly than the pulse current Ia from the discharging capacitor 39a.
[0042]
The steep rise of the discharge current of the flash lamp 22 increases the relaxation oscillation of the laser beam 11a output from the laser head unit 20. Therefore, first, a current Ib that gradually rises is applied to the flash lamp 22. At this time, the relaxation vibration generated in the laser beam 11a is small. Then, the pulse current Ia is applied to the flash lamp 22 when the relaxation oscillation due to the current Ib is stopped. Thereby, the relaxation vibration of the laser beam 11a can be made extremely small.
[0043]
Further, in the power supply unit 30, the simmer current is always discharged by the flash lamp 22, but this makes it possible to reduce the change in discharge impedance when the pulse current Ia is discharged by the flash lamp 22. As a result, the repetition of the pulse discharge can be easily increased in speed, and the advantages of increasing the light emission efficiency and extending the life of the flash lamp 22 can be obtained.
[0044]
As described above, for nondestructive inspection of a large structure, it is desirable to excite vibration having a maximum frequency of 10 to 100 kHz, and for that purpose, it is necessary to irradiate a laser beam having a pulse width of 10 to 100 μs. However, such a long-pulse laser beam cannot be obtained with a Q-switch type solid-state laser oscillator used in a conventional laser ultrasonic method.
[0045]
FIG. 7 is a diagram showing a waveform of laser light obtained by normal oscillation without using a Q switch in a Q switch type solid state laser oscillator. The horizontal axis is time, and the vertical axis is light intensity. The half width of the laser beam envelope shown in FIG. 7 is 40 μs, and many spike-like pulses due to relaxation oscillation are generated in this laser beam. The intensity of relaxation oscillation is as large as 90% or more of the peak power, and the half width of each pulse is as short as several ns. Thus, if a Q-switch type solid-state laser oscillator that has been conventionally used in the laser ultrasonic method is diverted, a long-pulse laser beam cannot be obtained.
[0046]
FIG. 8 is a graph showing the waveform of the laser beam 11a obtained by the laser light source 11 having the configuration shown in FIGS. The horizontal axis represents time t [μs], and the vertical axis represents light intensity [a. u. ]. The laser beam 11a shown in FIG. 8 has a half width of 50 μs, and the intensity of relaxation vibration is suppressed to 50% or less of the peak power. Since the nondestructive inspection apparatus shown in FIG. 1 uses such a laser beam 11a, low frequency vibration having a maximum frequency of about 10 to 100 kHz can be excited efficiently.
[0047]
Next, the vibration isolation structure of the laser light source 11 will be described. FIG. 9 is a side view of the external casing of the power source unit 30 and the cooling unit 52 of the laser light source 11.
In the laser light source 11, the flash lamp 22 is caused to emit light by high-speed discharge of the discharge capacitor 39a. At this time, a strong electromagnetic force is generated from the discharging capacitor 39a. The laser light source 111a used in the conventional laser ultrasonic method is installed on the floor with a caster or the like, and does not have a special vibration-proof structure. However, this causes the laser light source 111a to vibrate greatly due to the electromagnetic force accompanying high-speed discharge. This vibration is transmitted to the laser interferometer 113 through the floor and air, and there is a problem that it becomes noise of the output signal of the laser interferometer 113.
[0048]
Therefore, the laser light source 11 of the nondestructive inspection apparatus shown in FIG. First, the case 39c that accommodates the discharging capacitor 39a and the casing 30a of the power supply unit 30 are conventionally formed of iron or the like, but the case 39c and the casing 30a are formed of a nonmagnetic material such as brass. Thereby, even if electromagnetic force generate | occur | produces with high-speed discharge, case 39c and the housing | casing 30a do not vibrate.
Further, as shown in FIG. 9, when the casing 30 a of the power supply unit 30 and the casing 52 a of the cooling unit 52 a are arranged on the floor 55, an anti-vibration rubber 54 is interposed. Thereby, even if the housings 30a and 52a vibrate, the vibrations are not transmitted to the floor 55.
Since the laser light source 11 has such an anti-vibration structure, vibration due to electromagnetic force accompanying high-speed discharge is suppressed. Therefore, since noise on the vibration waveform detected by the vibration detector 13 is reduced, the inspection accuracy is increased.
[0049]
Next, an experiment relating to a nondestructive inspection of a structure using the apparatus shown in FIG. 1 will be described.
First, a prototype of a long pulse YAG laser having the configuration shown in FIGS. 4 and 5 was prototyped as the laser light source 11. This laser light source 11 was able to oscillate with a pulse width of 50 μs and energy of 2.5 J using flash lamp light emission by high-speed discharge of a large-capacitance capacitor 39a. At the same time, the spread angle of the laser beam 11a could be limited to 10 mrad or less.
In a preliminary experiment using this laser light source 11, vibration with a frequency of 1 to 50 kHz can be generated efficiently, and a clear resonance spectrum of a 210 × 100 × 30 mm brick block or a 100 × 100 × 5 mm tile is not obtained. It was confirmed for the first time that it could be detected by contact. Further, since the laser beam 11a has a long pulse width, the peak power is small instead of large energy, and a complete nondestructive inspection can be performed without causing ablation damage on the surface of the member.
[0050]
Each experiment will be described in detail.
Experiment 1. Comparison of S / N ratio between the conventional laser ultrasonic method and the present invention:
A 210 × 100 × 30 mm brick block was prepared as a sample. The brick block is placed on a flat surface with the surface of 100 × 30 mm facing down. At this time, the brick block is not bonded to a flat surface and is in a completely peeled state. In this state, a 210 × 100 mm surface of the brick block is irradiated with pulsed laser light. At this time, an ultrasonic wave based on the excited vibration is received 1 m from the irradiation position, and a frequency analysis is performed to obtain a resonance spectrum.
[0051]
First, a laser beam 111a having a pulse width of 8 ns used in a conventional laser ultrasonic method was irradiated. FIG. 10 is a graph showing the vibration waveform of the brick block at this time. FIG. 10A shows a detection waveform by the laser interferometer 113, and FIG. The power spectrum obtained by Fourier transforming the detected waveform is shown. 10A and 10B, the waveforms when the energy of the laser beam 111a is 116 mJ, 87.8 mJ, 57.5 mJ, and 35.0 mJ, and the background waveform are shown in order from the top. .
As can be seen from FIG. 10A, the amplitude of the vibration velocity v of the brick block at this time is as small as 0.005 mm / s or less. In FIG. 10B, a resonance spectrum is seen in the vicinity of 5 kHz, but the amplitude of this resonance spectrum is small and is buried in the background noise.
[0052]
Next, a laser beam 11a having a pulse width of 50 μs from a prototype of the laser light source 11 was irradiated. FIG. 11 is a graph showing the vibration waveform of the brick block at this time. FIG. 11A shows the detection waveform by the vibration detector 13, and FIG. 11B shows the vibration waveform by the vibration detector 13. The power spectrum obtained by Fourier transforming the detected waveform with the computer 14 is shown. 11A and 11B show waveforms when the energy of the laser beam 11a is 2.08 J (peak power 42 kW) and 1.25 J (peak power 25 kW), and a background waveform (FIG. 11 ( B) only) is shown in order from the top.
[0053]
As can be seen from FIG. 11A, the amplitude of the vibration speed v of the brick block at this time is 0.03 to 0.05 mm / s, and the amplitude of the vibration speed v is about 10 times that of the conventional art. In addition, in FIG. 11B, a clear resonance spectrum is seen near 5 kHz. This resonance spectrum indicates that the brick block is in a peeled state. Therefore, it can be seen that the state of the structure to be inspected can be detected with high sensitivity by using the long-pulse laser beam 11a.
[0054]
FIG. 12 shows the peak power I of the laser beam 11a created based on FIG.₀  It is a graph which shows the relationship between the amplitude of the vibration speed v of a brick block. In order to obtain a clear resonance spectrum, it is desirable that the amplitude of the vibration velocity v detected by the vibration detector 13 is 0.01 mm / s or more. Therefore, when the vibration excited by the inspection object is detected 1 m away from the irradiation position, the peak power I₀  It can be seen from FIG. 12 that a laser beam 11a of 10 kW or more may be used. By using such a laser beam 11a, remote inspection can be performed accurately.
[0055]
Experiment 2. Detection of peeling:
FIG. 13 is a cross-sectional view for explaining the structure of the sample of this experiment. On a 300 × 300 × 60 mm concrete plate 61, a 98 × 98 × 5 mm porcelain tile (interior tile 100 corners) 62 is bonded by a resin blending cement 63 of 100 × 100 × 3 mm. However, a nylon sheet 64 of 30 × 30 × 0.3 mm is sandwiched in advance at the interface between the tile 62 and the cement 63, and artificial peeling is formed. Here, a portion where the nylon sheet 64 is sandwiched is called a peeling portion, and a portion where the nylon sheet 64 is not sandwiched is called a healthy portion.
[0056]
First, the surface of the tile 62 in the healthy part was irradiated with a laser beam 11a having a pulse width of 50 μs according to the prototype of the laser light source 11. FIG. 14 is a graph showing the vibration waveform of the healthy part at this time. FIG. 14A shows the detection waveform by the vibration detector 13, and FIG. 14B shows the vibration waveform by the vibration detector 13. The power spectrum obtained by Fourier transforming the detected waveform with the computer 14 is shown.
Subsequently, the surface of the tile 62 at the peeling portion was similarly irradiated with the laser beam 11a. FIG. 15 is a graph showing a vibration waveform of the peeling portion at this time. FIG. 15A shows a detection waveform by the vibration detector 13, and FIG. 15B shows a vibration waveform by the vibration detector 13. The power spectrum obtained by Fourier transforming the detected waveform with the computer 14 is shown.
[0057]
Comparing FIG. 14B and FIG. 15B, it can be seen that a strong resonance spectrum appears in the vicinity of 15 kHz in FIG. 15B. This resonance spectrum is based on vibrations induced by thermal expansion of the tile portion between the delamination and the surface. The presence or absence of peeling can be known from the presence or absence of this resonance spectrum.
[0058]
Next, five types of samples having different sizes of artificial peeling by the nylon sheet 34 were prepared. Table 1 is a table showing the size of the artificial peeling and the area ratio Ra of the artificial peeling. Here, the area ratio Ra of artificial peeling is obtained by (area of artificial peeling) / (area of tile 62).
[0059]
[Table 1]

[0060]
And the laser beam 11a with a pulse width of 50 microseconds by the prototype of the said laser light source 11 was irradiated in order to the tile 62 surface of the peeling part of each sample. FIG. 16 is a graph showing a power spectrum obtained by Fourier transforming the vibration waveform of the peeling portion of each sample at this time. In this graph, the power spectrum of a sample with small artificial peeling is shown in order from the bottom. The horizontal axis of this graph is the frequency f [kHz], and the vertical axis is the intensity. However, the intensity of the power spectrum of each sample having the size of artificial peeling of 50 × 50 mm, 69.3 × 69.3 mm, 84.9 × 84.9 mm is 1/100, 1/200, and 1/400, respectively. It is compressed.
[0061]
In each power spectrum, the resonance spectrum with an arrow is based on the vibration induced in the tile portion between the separation and the surface. It can be seen that the greater the separation, the lower the resonance frequency based on the separation.
FIG. 17 is a graph showing the relationship between the area ratio Ra [%] of artificial peeling of each sample and the resonance frequency f [kHz] based on the artificial peeling. Based on this graph, the area ratio of peeling can be estimated from the obtained resonance frequency.
This experiment 2 is an experiment using a sample in which an artificial peeling is provided between the tile 62 and the concrete plate 61, but a peeling crack almost parallel to the surface of the large structure can be detected in the same manner. The area ratio of cracks can also be estimated from the resonance frequency.
[0062]
Experiment 3. Thickness detection:
Two concrete plates of 300 × 300 × 60 mm and 250 × 250 × 50 mm are prepared. Samples (A) and (B) were prepared by adhering 98 × 98 × 5 mm porcelain tiles (100 squares for interior tiles) to these concrete plates with 100 × 100 × 3 mm resin-containing cement. However, in any samples (A) and (B), there is no separation between the tile and the concrete board.
The tile surfaces of these samples (A) and (B) were irradiated with a laser beam 11 a having a pulse width of 50 μs by the prototype of the laser light source 11. FIG. 18 is a graph showing the vibration waveforms of the samples (A) and (B) at this time, and FIGS. 18A and 18B show the vibration waveforms of the samples (A) and (B), respectively. FIGS. 18C and 18D show power spectra obtained by Fourier transforming the vibration waveforms of the samples (A) and (B), respectively.
[0063]
As shown in FIG. 18C, the resonance frequencies of the vibration induced by the thermal expansion of the sample (A) are 5.3 kHz and 6.0 kHz. Further, as shown in FIG. 18D, the resonance frequencies of vibration induced by thermal expansion of the sample (b) are 6.2 kHz and 7.5 kHz. The difference in resonance frequency is due to the difference in the thickness of the concrete plates of samples (A) and (B). Therefore, the thickness of the concrete board can be estimated from the obtained resonance frequency.
This technology can also be applied to large structures. For example, the thickness of the tunnel wall can be estimated by the same method.
[0064]
As is clear from the experimental results of the above experiments 1 to 3, by using the laser light 11a having a pulse width of 10 to 100 μs and suppressed relaxation vibration, excited vibration is also applied to a large structure. It can be detected with high sensitivity. Further, the peak power I of the laser beam 11a₀  By setting the power to 10 kW, vibration excited even 1 m away from the irradiation position can be detected with high sensitivity. Therefore, with the apparatus shown in FIG. 1, the soundness of a large structure can be remotely and highly sensitively nondestructively inspected. In addition, the nondestructive inspection by the apparatus shown in FIG. 1 can be performed easily because it can be performed without manual operation and without contact.
The nondestructive inspection apparatus shown in FIG. 1 is not limited to tunnels, and can be widely applied to nondestructive inspection of large structures such as transportation, power generation facilities, and residential buildings.
[0065]
For example, when the inside of the wall has a two-layer structure in the depth direction, the portion shown in FIG. 1 can detect an incompletely bonded interface between the two layers. When such a portion is irradiated with a long pulse laser beam 12a, stress is generated on the surface layer due to thermal expansion, and the interface between the two layers is peeled off, and vibration is excited in the irradiated region on the same principle as described above. Because. The same applies to the case where a reinforcing bar is embedded in the column and the adhesion between the column concrete and the reinforcing bar is incomplete.
Moreover, when the thickness of the surface layer of a large structure having a two-layer structure is about 1 cm, the thickness of the surface layer can be accurately measured even if the interface between both layers is completely adhered. When the surface temperature rises by about several tens of degrees centigrade by irradiating a long pulse laser beam 12a of about 1 J, the adhesion between the interfaces of both layers temporarily weakens, and the irradiation region of the surface layer is the same as when the interfaces of both layers peel off. Because it vibrates.
[0066]
As described above, with the nondestructive inspection apparatus shown in FIG. 1, as the internal structure of the large structure, a crack of about 1 to 50 cm and a thickness of the structure of about 10 to 100 cm formed almost parallel to the surface of the structure. It is possible to measure the thickness of the surface layer of a structure having a thickness of about 1 cm, a portion where the adhesion of the interface is incomplete in a structure having a multilayer structure or a structure in which another member is embedded.
The nondestructive inspection apparatus shown in FIG. 1 can also be applied to a mobile inspection system that can measure the presence and absence of concrete deterioration and cracks of several meters or more and the shape thereof quickly and accurately. This is thought to contribute to the dramatic improvement of the safety of large structures that are currently problematic in various fields, and at the same time, to create a new inspection industry.
[0067]
【The invention's effect】
As described above, in the present invention, the pulse width of the pulsed light used for the inspection is set to 10 to 100 μs, and the relaxation oscillation is suppressed to 50% or less of the peak power. As a result, vibration of about 10 to 100 kHz with small attenuation in the large structure can be efficiently excited, and thus the soundness of the large structure can be measured with high sensitivity. Moreover, in this invention, the peak power of the pulsed light used for a test | inspection shall be 10 kW or more. Thereby, the soundness of a large-sized structure can be measured remotely.
In the present invention, the light source is composed of a lamp-excited normal oscillation solid-state laser oscillator. In addition, a preliminary discharge circuit is provided in the lamp excitation power supply unit so that the excitation lamp is always discharged. As a result, the change in the discharge impedance when the pulse current is discharged by the excitation lamp can be reduced, so that the repetition of the pulse discharge can be easily increased in speed, the luminous efficiency is increased, and the life of the excitation lamp is extended. It is done.
[0068]
In the present invention, the case of the high-speed discharge circuit of the light source and the casing of the power source unit of the light source are formed of a non-magnetic material. In addition, the power source unit and the cooling unit housing of the light source have an anti-vibration structure. Thereby, the vibration by the electromagnetic force accompanying a high-speed discharge is suppressed. Therefore, the noise on the vibration waveform detected by the vibration detecting means is reduced, and the inspection accuracy is increased.
In the present invention, the spread angle of the pulsed light is limited to 10 mrad or less. As a result, the directivity of the pulsed light is increased, so that inspection data for only a desired region can be acquired.
In the present invention, a large area inspection can be easily performed by scanning the irradiation position of the pulsed light. Here, high-speed scanning is possible by using an acousto-optic modulator as the scanning means.
[Brief description of the drawings]
FIG. 1 is a block diagram showing the configuration of an embodiment of a nondestructive inspection apparatus for structures according to the present invention.
FIG. 2 is an explanatory diagram of the principle of generation of ultrasonic waves.
FIG. 3 is a graph showing a temperature distribution of an object surface layer.
FIG. 4 is a configuration diagram of a laser light source.
FIG. 5 is a circuit diagram showing a configuration of a power supply unit of a laser light source.
FIG. 6 is a graph showing current (excluding simmer current) supplied to the flash lamp.
FIG. 7 is a diagram showing a waveform of a laser beam obtained by normal oscillation without using a Q switch in a Q switch type solid-state laser oscillator.
8 is a graph showing a waveform of a laser beam obtained by a laser light source 11 having the configuration shown in FIGS. 4 and 5. FIG.
FIG. 9 is a side view of an external housing of a power source unit and a cooling unit of a laser light source.
FIG. 10 is a graph showing a vibration waveform of a brick block when a laser beam having a pulse width of 8 ns is irradiated.
FIG. 11 is a graph showing a vibration waveform of a brick block when a laser beam having a pulse width of 50 μs is irradiated.
FIG. 12 is a graph showing the relationship between the peak power of laser light and the amplitude of the vibration speed of a brick block.
13 is a cross-sectional view for explaining the structure of a sample in Experiment 2. FIG.
FIG. 14 is a graph showing a vibration waveform of a healthy part when a laser beam having a pulse width of 50 μs is irradiated.
FIG. 15 is a graph showing a vibration waveform of a peeled portion when a laser beam having a pulse width of 50 μs is irradiated.
FIG. 16 is a graph showing a power spectrum based on a vibration waveform of a peeling portion of each sample when laser light having a pulse width of 50 μs is irradiated.
FIG. 17 is a graph showing a relationship between an area ratio of artificial peeling of each sample and a resonance frequency based on the artificial peeling.
FIG. 18 is a graph showing vibration waveforms of samples (A) and (B) when a laser beam with a pulse width of 50 μs is irradiated.
FIG. 19 is a block diagram showing a configuration example of an inspection apparatus for detecting a defect inside an object by a conventional laser ultrasonic method.
FIG. 20 is a diagram showing a waveform of a laser beam used in a conventional laser ultrasonic method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Tunnel, 2 ... Tunnel surface, 3 ... Crack, 4 ... Plate-like part, 4a ... Ultrasound, 10 ... Nondestructive inspection device of structure, 11 ... Laser light source, 11a, 12a ... Laser light, 12 ... Sound Optical modulator, 13 ... Vibration detector, 13a, 14a ... Data, 14 ... Computer, 14s, 14t ... Control signal, 15 ... Display device, 20 ... Laser head, 21 ... Laser rod, 22 ... Flash lamp, 23 ... Condenser, 24a ... Total reflection mirror, 24b ... Output mirror, 25 ... Expander, 25a ... Convex lens, 25b ... Concave lens, 26 ... Polarizer, 30 ... Power supply unit, 30a, 52a ... Case, 31 ... Shimmer power supply, 32 ... resistor, 33 ... diode, 34 ... trigger pulse generator, 35 ... transformer, 36 ... electrode, 37 ... main power supply for charging, 38a, 38b, 41 ... inductor, 3 a, 39b ... discharge capacitor, 39c ... case, 40a, 40b ... switching element, 51 ... laser oscillation control part, 51a, 51b ... light emission command signal, 52 ... cooling part, 54 ... anti-vibration rubber, 55 ... floor, 61 ... concrete board, 62 ... magnetic tile, 63 ... resin-mixed cement, 64 ... nylon sheet.

Claims (13)

The pulse width is 100 microseconds or less than 10 microseconds, the peak power is higher than is and relaxation oscillation generates a pulsed light with suppressed 10 kW, a first irradiating the pulsed light on the front surface of the structure Process,
A second step of detecting vibrations generated by light energy absorption under and on the surface of the structure in the region irradiated with the pulsed light;
A third step of generating information on the internal structure of the structure based on the detected waveform of the vibration ,
The pulsed light is laser light generated by a preliminary discharge circuit that constantly supplies a predetermined direct current to the excitation lamp and a high-speed discharge circuit that supplies a pulsed current to the excitation lamp,
The preliminary discharge circuit and the high-speed discharge circuit is non-destructive inspection method of a structure which is characterized that you have been accommodated in a housing having a housing or vibration isolation structure formed of a nonmagnetic material. In the nondestructive inspection method of the structure according to claim 1,
In the first step, the non-destructive inspection method for a structure is characterized in that relaxation oscillation of the pulsed light is suppressed to 50% or less of the peak power. In the nondestructive inspection method of the structure according to claim 1 or 2,
In the first step, the surface of the structure is irradiated with the pulsed light in a non-contact manner,
In the second step, the vibration is detected in a noncontact manner on the surface of the structure. In the nondestructive inspection method of the structure according to any one of claims 1 to 3,
In the third step, the detected vibration waveform is subjected to Fourier transform to obtain a resonance frequency of the vibration, and at least one of an area of a gap formed in the structure and a thickness of the structure is determined as the resonance of the vibration. A non-destructive inspection method for a structure characterized by estimating from a frequency. In the nondestructive inspection method of the structure according to any one of claims 1 to 4,
A nondestructive inspection method for a structure, wherein in the first step, the irradiation position of the pulsed light is scanned. A light source that emits pulsed light onto the surface of the structure;
Vibration detecting means for detecting vibration generated by light energy absorption under and on the surface of the structure in the region irradiated with the pulsed light;
Computation processing means connected to the output side of the vibration detection means and generating information related to the internal structure of the structure based on the vibration waveform detected by the vibration detection means,
The light source has relaxation vibration suppressing means for suppressing relaxation vibration of the pulsed light,
The pulse width of the pulsed light is 10 microseconds or more and 100 microseconds or less,
Peak power of the pulsed light state, and are more 10 kW,
Further, the light source is
A laser oscillator of a normal oscillation solid-state laser that obtains laser output by exciting the laser medium with light from the excitation lamp;
A power source unit including a preliminary discharge circuit that constantly supplies a predetermined direct current to the excitation lamp, and a high-speed discharge circuit that supplies a pulse current to the excitation lamp;
The fast at least one case and the housing of the power supply unit of the discharge circuit, nondestructive inspection device of a structure which is characterized that you have formed a non-magnetic material. A light source that irradiates the surface of the structure with pulsed light;
Vibration detecting means for detecting vibration generated by light energy absorption under and on the surface of the structure in the region irradiated with the pulsed light;
Computation processing means connected to the output side of the vibration detection means and generating information relating to the internal structure of the structure based on the vibration waveform detected by the vibration detection means,
The light source has relaxation vibration suppressing means for suppressing relaxation vibration of the pulsed light,
The pulse width of the pulsed light is 10 microseconds or more and 100 microseconds or less,
The peak power of the pulsed light is 10 kW or more,
Further, the light source is
A laser oscillator of a normal oscillation solid-state laser that obtains laser output by exciting the laser medium with light from the excitation lamp;
A power supply unit comprising: a preliminary discharge circuit that constantly supplies a predetermined direct current to the excitation lamp; and a high-speed discharge circuit that supplies a pulse current to the excitation lamp;
A cooling unit for cooling the laser medium and the excitation lamp,
The non-destructive inspection apparatus for a structure according to claim 1, wherein the power supply unit and the cooling unit have a vibration-proof structure . The nondestructive inspection device for a structure according to claim 6 or 7,
The light source of the relaxation oscillation suppression means, non-destructive inspection method of a structure, characterized in that the relaxation oscillation of the pulse light is a means for inhibiting 50% or less of the peak power of the pulsed light. In the nondestructive inspection device for a structure according to any one of claims 6 to 8,
The light source is means for irradiating the surface of the structure with the pulsed light in a non-contact manner;
The non-destructive inspection apparatus for a structure, wherein the vibration detecting means is means for detecting the vibration without contact with the surface of the structure. In the nondestructive inspection device for a structure according to any one of claims 6 to 9 ,
The non-destructive inspection apparatus for a structure according to claim 1, wherein the relaxation vibration suppressing means of the light source is a means for gradual rising of a pulse current from the high-speed discharge circuit . In the nondestructive inspection device for a structure according to any one of claims 6 to 9 ,
The non-destructive inspection apparatus for a structure , wherein the light source has a spread angle control means for limiting a spread angle of the pulsed light to 10 mrad or less . In the nondestructive inspection device for a structure according to any one of claims 6 to 9 ,
A non-destructive inspection apparatus for a structure, comprising: a scanning unit that is disposed on an output side of the light source and scans an irradiation position of the pulsed light . The nondestructive inspection apparatus according to claim 1 2 Structure according,
The pulsed light is linearly polarized light,
The non-destructive inspection apparatus for a structure is characterized in that the scanning means is an acousto-optic modulator .

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